1. Field of the Invention
This invention relates to a magnetic detector using reluctance segments and, more particularly, to a magnetic detector capable of detecting a change in the magnetic field caused by the motion of a magnetic moving body coupled to an object that is to be detected.
2. Description of the Related Art
In recent years, a car-mounted rotation sensor and the like are utilizing a magnetic detector which detects a change in the magnetic field acting on the reluctance elements by constituting a bridge circuit by forming electrodes at the ends of the reluctance segments, connecting a constant-voltage and constant-current source between the two opposing electrodes of the bridge circuit, and converting a change in the resistance of the reluctance segments into a change in the voltage, and various proposals have been made so far.
A conventional magnetic detector has been disclosed in, for example, JP-A-2005-156368 according to which a detector unit in the processing circuit is constituted by at least six magnetic/electric conversion elements. According to this magnetic detector as will be understood from the description of the specification thereof, the magnetic/electric conversion element is constituted by at least six segments that are symmetrically arranged along the direction in which the magnetic moving body rotates maintaining a predetermined pitch with respect to a center line of the magnet that intersects the direction of rotation at right angles, first and second bridge circuits are formed including at least two pairs of segments which are so arranged that the centers of pitch are symmetrical to each other with respect to the center line of the magnet so as to produce outputs accompanying the rotation of the magnetic moving body, and a third bridge circuit is formed including at least a pair of segments having a center of pitch on the center line of the magnet so as to produce an output accompanying the rotation of the magnetic moving body, wherein a phase difference of ¼ period is imparted to between the output of the third bridge circuit and the differential output of the first and second bridge circuits, so that the direction in which the magnetic moving body is rotating can be correctly detected even when the comparison level of the comparator circuit has fluctuated or even when the output of the amplifier circuit has fluctuated.
In the above conventional magnetic detector, however, it has been known that a temperature offset occurs in the neutral point output due to a change in the environmental temperature being caused by a difference in the temperature characteristics of the reluctance segments constituting the bridge circuits. FIG. 5 is a diagram of operation waveforms illustrating the states at that time, wherein FIG. 5A represents resistances of the reluctance segments, FIG. 5B represents the neutral point outputs A and B of the first and second bridge circuits, FIG. 5C represents outputs OP1 and OP2 of the amplifier circuits, FIG. 5D represents outputs Vout1 and Vout2 of the comparator circuits, and FIG. 5E represents final outputs 1 and 2 of the output circuits.
In an environment where, for example, the temperature characteristics varies greatly, as will be obvious from the drawing, the waveforms of the neutral point output A and of the neutral point output B differ between a high temperature state (HOT) and a room temperature state (R.T) as shown in FIG. 5B, and the effect appears on the comparator circuit output Vout1 (see FIG. 5D). A temperature offset T1 further appears on the final output 1, too. As a result, the precision of detection of the magnetic detector decreases and the direction of rotation cannot be detected correctly.